Aluminum base alloy plate



United States Patent M 3,333,989 ALUMINUM BASE ALLOY PLATE Melvin H.Brown, Leechburg, Bernard W. Lifka, New

Kensington, and Stillman D. Pitts, Pittsburgh, Pa., assignors toAluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Filed Feb. 5, 1965, Ser. No. 430,743 Claims.(Cl. 14812.7)

ABSTRACT OF THE DISCLOSURE- Improved aluminum alloy plate containing 3to 6% copper, 0.8 to 3% magnesium and 0.3 to 1% manganese and exhibitingimproved short transverse properties characterized by an elongation of 3to 6%, a tear resistance unit propagation energy of at least 100 can beprovided by a fabrication cycle which includes an A forging reduction ofelongated stock which is then drawn out and hot rolled to form theplate. In addition, the magnesium content is controlled in relation tothe copper and manganese content in accordance with the followingequation:

Mg min.=0.32 Cu where Mn is not greater than 0.5%, Mg min.=0.2+0.32Cu0.4 Mn where Mn is greater than 0.5%

to impart substantial immunity to stress corrosion in the shorttransverse direction.

This invention relates to aluminum base alloy plate having improvedshort transverse tensile elongation and resistance to stress corrosion.

For some time, heat treatable aluminum base alloy plate containingcopper, magnesium and manganese has found considerable acceptance forvarious structural members. Such an alloy known in the art containsnominally 4.5% copper, 1.5% magnesium and 0.6% manganese and carries theAluminum Association designation of 2024 alloy. This alloy plate, atleast in relatively thin section, is noted for its very good strength toweight ratio, its outstanding toughness and tear resistance and its goodresistance to general and stress corrosion effects. It has beencommercially produced by conventional rolling procedures wherein aningot having a rectangular transverse cross section of say about 14 to16 inches by 45 .to 50 inches is hot rolled to the desired thickness ofplate. Plate so produced is marked by a rather limited level ofductility across the short transverse, or thickness, dimension,generally exhibiting short transverse tensile elongation values as lowas /2% and very rarely over 2%. Also notable is the relatively low levelof toughness or tear resistance exhibited across the plate thickness.One reliable measure of toughness or tear resistance is the Kahn teartest described in the American Society for Testing Materialspublication, Materials Research and Standards, vol. 4, No. 4, April1964. In this test conventionally hot rolled plate exhibits a shorttransverse unit propagation energy of about 80 inch pounds per squareinch compared to long transverse and longitudinal levels of about 110and 150 respectively. For

3,333,989 Patented Aug. 1, 1967 relatively thin plate this deficiency isof little consequence since such is generally not stressed in thatdirection. Potential new applications for the alloy, however, indicatethat relatively thick plate, eg 2" to 6", would olfer substantiallyincreased utility if its short transverse ductility and toughness ortear resistance could be improved so that substantial tensile stressesmight be imposed across its thickness in multi-directionally stressedmembers. One known manner for improving short transverse mechanicalproperties in plate is to forge the ingot to a suitable shape prior tothe hot rolling operation. For example, the forging practice may consistof reducing the alloy type described, this technique has produced to 10inches thick and then hot rolling this slab. With thea lloy typedescribed, this technique has produced transverse properties that areinconsistent and the elongation still rarely exceeds 2% and very little,and more often, no consistent improvement in toughness or tearresistance. As a futher complication a decrease in resistance to stresscorrosion is associated in the art with such preliminary forging methodswhich consequently are generally viewed with caution.

In accordance With the invention it has been found that by the practiceof a special preliminary forging operation highly repeatable andreliable improvements in short transverse tensile elongation andtoughness or tear resistance are realized such that the finished plateexhibits an elongation level of about 3% to 6% and a unit propagationenergy of at least and more often about to in. 1b./sq. in. Further byvirtue of certain critical composition limits the plate exhibitsentirely satisfactory resistance to stress corrosion, even where thestress is applied across the plate thickness.

Accordingly it is a primary object of the invention to provide platemembers of the described aluminum base alloy type which arecharacterized by a high level of elongation, resistance to stresscorrosion and tear resistance in the short transverse direction.

Another object of the invention is to provide forged and hot rolledplate of the aluminum base alloy type described having a high level ofresistance to stress corrosion in the short transverse direction.

Yet another object of the invention is to provide a method of producingforged and hot rolled plate of the described aluminum base alloy typehaving improved isotropy of mechanical properties and resistance tostress corrosion.

Another object of the invention is to provide a method of producingplate of the aluminum base alloy type described, the plate exhibiting ahigh level in elongation, resistance to stress corrosion and tearresistance in the short transverse direction.

Other objects will, in part, be obvious and will, in part, appearhereinafter.

Basically the invention resides in imparting to the alloy body or ingot,prior to hot rolling, a severe forging upset whereby an ingot or otherbody having a greater length than the cross sectional dimension iscompressed in an plate thickness, solution heat treated and artificiallyaged.

While plate so produced will exhibit greatly improved short transverseproperties, certain critical composition limits must be imposed over andabove those currently applied by the art to the described type aluminumbase alloys to avoid an adverse effect in resistance to corrosion. Thislimit is explained in more detail hereinafter but in its most simpleterms is stated that the magnesium content generally must be at least0.32 times the copper content. From a composition standpoint the alloyconsists essentially of aluminum and, on a weight basis, 3 to 6% copper,1 to 3% magnesium and 0.3 to 1% manganese together with impurities. Apreferred composition is one consisting essentially of aluminum, 3.5 to5% copper, 1 to 2% magnesium and 0.3 to 1% manganese together withimpurities. Of course, the critical magnesium and copper relationshipmust be followed in order to avoid serious impairment of resistance tostress corrosion in the short transverse direction. The plate which isbenefited by the controlled composition and forging practice generallyranges in thickness from 1" to 8", but more often from 2" to 6" in mostapplications where short transverse properties are of substantialsignificance.

While the initial forging step is relatively simple, it yieldsunexpected results in combination with subsequent hot working operationsin that highly repeatable improvements in short transverse mechanicalproperties are achieved when applied to bodies of the above-describedalloys. As suggested above, this upset is accomplished by compressing arelatively long ingot in an axial direction and further processing theingot such that the length in an axial direction becomes the shorttransverse dimension of the final plate. In view of such a drasticreduction in the direction which eventually becomes the plate thickness,it is entirely unexpected that such improvements in short transverseproperties would result since the art generally associates severe shorttransverse reductions with impairing properties in that direction andsuch a practice is therefore foreign to the art of producing plate ofthe described alloy type. The alloy body or stock to which the forgingsteps are applied is generally a relatively large continuously castingot which has its surface discontinuities removed by scalping and itsends cropped to remove end defects. However, such is not intended as alimit on the invention which includes the application of the describedforging steps to any body whose shape and dimensions are consistent withthe method. Hence reference herein to an alloy body is intended toinclude both ingot and other suitable stock. Prior to forging, the bodyis gradually heated to a temperature above 800 F. but generally short ofincipient melting, and most often between 900 and 925 F., and thensoaked at that temperature for a prolonged period principally tohomogenize its internal structure and also relieve some of the internalstresses introduced in casting. The soaking time is generally over hoursand more often between and hours, although much longer soak times of 50hours and more are often used.

The degree to which the alloy body is compressed or upset in an axialdirection is at least 60% and preferably over 75% of the originallength. An additional factor which has a very significant effect uponthe degree of upset achieved is the ratio of the length of the body tothe base dimensions. The base dimension is considered the diameter forround bodies and the mean base dimension for rectangular or other shapedbodies. For instance, a body 20" by 40" in cross section is consideredto have a base dimension of the same as a 30" square or round body. Thelength to base ratio should be at least 2:1 and is preferably at least2' /z:1. Thus in accordance with the invention, an alloy body is upsetby a forging operation, at a temperature which most often ranges from700 to 900 R, which decreases the length of a body having a length tobase ratio of at least 2:1, and preferably at least 2 /211, by at least60%,

and preferably at least 75%, to form a biscuit the height of which isnot greater than 40%, preferably not over 25%, the length of the initialbody. An upset in this, the axial, direction is generally termed an Aupset; that is, an upset compressing the body length, referred to hereas the A dimension. The forged biscuit is generally reheated to atemperature of about 800 to 850 F., flattened slightly on its sides andthen further compressed in the original A direction on a forging press,hammer, or the like to form in an elongated slab, the thickness of whichis not more than three fourths the height (thickness) of the biscuit andmore often ranges from A to /2 the biscuit height. Since forging androlling facilities are not usually beside each other, the forged slab iscooled to room temperature and reheated when it is to be rolled. Theelongated slab is hot rolled, generally at 800 to 900 F. to the finalplate thickness. The hot rolling generally reduces the thickness of theslab by 20% or more, but most often the thickness reduction ranges from35% to Thus the initial ingot length is converted to the final platethickness. Of course, as indicated above, the forging operations,together with the hot rolling operations, are performed at elevatedtemperatures within the approximate range of 600 to 900 F., and moreoften between 700 and 900 F., the temperature generally diminishingsomewhat between the start and completion of any hot working operation,and hence temperature generally refers to the level at the start of theoperation. The ranges set forth above for the individual operations arethose which have been used most often in practicing the invention. Ifthe stock cools excessively during a hot working operation, such may beinterrupted for purposes of reheating in accordance with the generalpractice in the art.

To develop a high strength and hardness the hot rolled plate is solutionheat treated, for example, at about 800 to 950 F., or preferably atabout 900 to 925 F., for about one to twelve hours or more depending onthe plate thickness. This time is expressed functionally as being ofsufficient duration to effect substantial solution of the soluble alloyconstituents. The plate is then rapidly quenched, generally by sprayquenching especially where the plate is of relatively thick crosssection, and artificially aged, at about 300 to 400 F. for about 6 to 24hours, but more often within narrower ranges of 360 to 390 F. for 10 to13 hours, by methods currently practiced in the art. The artificialaging step is preferably preceded by a mechanical stress relieftreatment by means of working at substantially room temperaturesufficiently to efifect a 1 /2 to 3% permanent stretch for example, theplate is often cold stretched to effect a permanent stretch of about 1%It might be noted that the solution heat treatment, quench, coldstretching and artificial aging operations as performed in the practiceof the invention are much the same as those currently practiced in theart of treating 2024 type aluminum alloy. Thus temperature levels,holding times at temperature and other factors can be determined forspecific plate thicknesses by those practicing the art and need not befurther developed here.

Plate produced as set forth above exhibits short transverse tensileelongation generally ranging from 3% to 6% and tear resistancecharacterized by unit propagation energy of to in. lb./sq. in. thusrendering the plate entirely suitable for design applications where itis stressed in that direction. As the mechanical properties in the otherdirections, long transverse and longitudinal, are not impaired, and theshort transverse properties are improved, the plate is considered asexhibitmg more isotropy, or less directionality, in tensile andelongation properties than the hot rolled plate of the prior art.Typical average mechanical strength values of conventional hot rolled2024 alloy plate compared with the values of special forged and rolledplate of the invention are listed in Table I where the abbreviation Ks.i. represents 1000 pounds per square inch. The plate thicknessesranged from 1%" to 5" and the plates of both series were given the samesolution heat treatment, quench, stretch and artificial aging.

Long Transverse It is quite apparent that the specially forged and hotrolled plate exhibits markedly improved short transverse elongation,more than twice that of the conventionally hot rolled plate. Theattendant significant improvements in short transverse strength are alsonoteworthy, the result being that the tensile properties of thepreforged plate are more isotropic, or less directional, than theconventional plate. Also, not only are the longitudinal and longtransverse properties unimpaired, they are slightly improved. Thus thetensile tests indicate a general upgrading of mechanical properties .inall directions but particularly the short transverse direction which isnow in more 'comformance with the others. One additional item ofinterest observed in these tests is that the mechanical properties werenot affected to any'substantial extent by alloying variations anywherewithin 2024 alloy composition limits and these results would then apply,generally speaking, to the entire composition range for this alloy.

Table II lists typical average tear test results comparing conventionaland specially forged and hot rolled plates within 2024 alloy compositionlimits. The plate temper and thickness ranges are the same as for TableI. The Kahn type of test, referred to earlier in this description, isconsidered to offer a good measure of the toughness of an article inthat the resistance to initiating and propagating a crack at a stressednotch is measured.

As with the tensile properties, the specially preforged plate exhibits amarked improvement in toughness as measured by the tear test. Also, aswith the tensile tests, tear resistance was not significantly affectedby variations in composition. It can be seen that the over-all toughnessis considerably upgraded, the short transverse performance beingimproved to a level comparable to the long transverse .and longitudinalvalues for conventional plate. This improved short transverse toughnessor tear resistance enables the plate to perform where conventionalhotrolled plate was previously considered inadequate.

The sequence of steps in carrying out the invention is illustrated inthe following example.

TABLE II.TEAR PROPERTIES IN DIFFERENT DIRE OTIONS Longitudinal TearStrength, Ratio: Unit Propaga- K s.i. Tear Strength tion Energy YieldStrength 'l 111-) Hot rolled 59. 4 0.96 145 Forged 71. 4 1. 15 260 LongTransverse Hot rolled 54. 2 0.89 Forged 66. 6 1. 06 200 Short TransverseHot rolled l 41. 6 0. 71 80 Forged 52. 2 0.87

Example I A round ingot can be cast by any of the conventionalcontinuous casting methods and is composed of an aluminum base alloynominally containing 4.8% copper, 1.5% magnesium and 0.5% manganesetogether with incidental impurities. In this case the ingot has adiameter of about 37 inches and a length of about 100 inches afterscalp-ing and cropping. The ingot is gradually heated to about 915 F. ina conventional furnace and then soaked at that temperature for about 24hours to homogenize its internal structure and prepare it for hotWorking. Upon removal from the furnace and before the temperature dropsto 800 F., it is placed on end in a large forging press .and compressedin the A direction, the 100 inch long ingot being reduced to a biscuitabout 36 inches high and about 75 inches in its swelled diameter. Thisbiscuit is reheated to a temperature of a little over 850 F. squaredslightly by compression on its sides which also reduces the lateralswelling at mid-height and then further compressed by drawing down on aforging press, again in the original A direction, to form an elongatedslab about 14 inches thick, 50 inches wide and to inches long. The slabis then reheated to about 850 to 900 F. and hot rolled to a'thickness ofabout 4 inches. Next, the plate is solution heat treated at a metaltemperature of slightly over 900 F. and a soak time of about 4 to 5hours and then quenched. For this thick plate it is preferable to sprayquench the plate to assure reasonably rapid cooling throughout itsthickness. The plate is cold stretched at room temperature to effect a 1/2 to 3 permanent stretch principally to relieve some of the internalstresses. The plate is then artificially aged by a ten to thirteen hoursoak at .a metal temperature of about 370 to 380 F. This plate is testedfor short transverse mechanical properties and is found to exhibit amarked improvement over conventionally hot rolled plate. Averageelongation and unit propagation energy values for this plate are alittle over 4% and about 125 in. lb./sq. in. as contrasted to averagevalues of about 1.4% and 75 to 80 for conventionally hot rolled 4"plate. 7 As indicated earlier in this description, the basic alloycomposition embodied in the invention consists essentially of aluminum,3 to 6%, preferably 3.5 to 5%, copper l to 3%, preferably 1 to 2%,magnesium and 0.3 to 1% manganese together with impurities. Generallyspeaking the impurity limits associated with the described type alloyspreferably apply to the practice of the invention. Thus the followingmaximum impurity limits are preferably followed: silicon 0.5%, iron 0.5chromium 0.1% and zinc 0.25%. Another composition limit, over the abovethat practiced in the prior art, which is required by the practice ofthe invention is the highly critical relation between the principalalloying constituents, copper and magnesium. Stated briefly the minimummagnesium content is equal to 0.32 times the copper content except thatthis minimum is lessened slightly where manganese exceeds 0.5% asexplained hereinafter; preferably the magnesium content is at least 0.2%over and above this minimum. Within the broader range (minimum Mg=0.32Cu) the alloy plate will exhibit very good resistance to stresscorrosion for loads up to and slightly exceeding 50% of the yieldstrength, a stress level of about 30 K s.i. based on a nominal yieldstrength of 60 K s.i. for 2024 aluminum alloy. The additional 0.2%magnesium, in accordance with the preferred practice of the invention,raises the permissible stress level to and above 75% of the yieldstrength, that is a stress level of about 45 K s.i. based on the nominal60 K s.i. yield strength for 2024 alloy. These results are based uponnumerous standard accelerated tests of the alternate immersion type in a3.5% sodium chloride aqueous solution. The results indicate thatspecimens within the broad alloy composition limit withstand 50% yieldstress application for at least thirty days. This is considered in theart as a reliable indication of indefinite life under ordinaryatmospheric conditions at this stress level, or in other Words,substantial immunity to stress corrosion. Likewise the additional 0.2%magnesium increases the permissible stress application to 75% and moreof the yield stress level with substantial immunity to stress corrosion.

An important modification to the basic magnesium and copper relationshipoccurs where manganese exceeds 0.5%. For this relatively high manganesecontent, the minimum magnesium necessary to maintain substantialimmunity to stress corrosion is slightly lessened. Accordingly, thefollowing equations govern the minimum magnesium content in accordancewith the invention where the stress levels do not exceed 50% of theyield strength by a substantail amount:

(1) min. Mg=0.32 Cu where Mn does not exceed 0.5 (2) min. Mg=0.2+0.32Cu0.4 Mn Where Mn exceeds In the preferred practice of the inventionwhere the applied stress can equal and even slightly exceed 75 of theyield stress, the equations become:

(3) min. Mg=0.2+0.32 Cu where Mn does not exceed (4) min. Mg=0.4+0.32Cu-0.4 Mn where Mn exceeds The following example illustrates the benefitderived from controlling the alloy composition in accordance withEquations 2 and 4 emphasizing its importance in combination with thespecial forging operation.

Example 11 Ingots were prepared from aluminum base alloys havingcompositions within the following limits: 4.15 to 4.6% copper, 1.4 to1.7% magnesium, 0.5 to 0.65% manganese, and impurities Within themaximum stated amounts: silicon 0.15%, iron 0.25%, chromium 0.10%, zinc0.20%, and nickel 0.05%. In some instances up to 0.05% titanium and upto 0.002 boron are added for grain refining purposes. It is noteworthythat this composition range inherently satisfies the minimum magnesiumrequirements of Equations 1 and 2. The cropped and scalped ingotsgenerally had a length of over 110 inches and a diameter around 37inches. These ingots were soaked at about 900 to 925 F. for about 20hours to homogenize their internal structure and prepare them for hotforging. They were then stood on end and compressed in the A directionin a large forging press at about 800 F. to form biscuits having heightdimensions of 32 inches, which represented a reduction of about 70%. Thetemperature during this forging step did not fall significantly below700 F. and generally ran between 750 and 800 F. The resulting biscuitswere then reheated to about 800 to 850 F. and drawn down on a forgingpress to form elongated slabs about 150 long, 60 wide and having athickness of about 15 inches. These elongated slabs were reheated forhot rolling to about 875 F. and rolled to a final thickness generallyranging from 3 to 4 /2 inches. The plates were solution heat treated bysoaking them for 4 to 5 hours at about 910 to 925 F., spray quenched,cold stretched at room temperature to effect about a 1%% permanentstretch and artificially aged for about 11 hours at a temperature ofabout 370 to 380 F. The resulting plate exhibited short transversetensile elongation values ranging from 3% to 8%, most often 4% to 6%,and a general improvement in toughness and tear resistance characterizedby unit propagation energy levels consistently over 110 inch pounds persquare inch. The plate where the magnesium content exceeded that setforth in Equations 3 and 4 exhibited substantial immunity to stresscorrosion at stress levels of up to and over 45,- 000 p.s.i. yield) asevidenced by the accelerated thirty day alternate immersion test in 3.5%NaCl solution. The other plates, containing magnesium at a lower levelthat exhibited like results provided the applied stress did notsubstantially exceed 30,000 p.s.i. (50% yield).

These results may be contrasted with those obtained for plates ofaluminum base alloys containing 4.2 to 4.8% copper, 1.2 to 1.5%magnesium and 0.30 to 0.45% manganese wherein the magnesium content wasless than 0.32 times the copper content. The plate was produced byconventional hot rolling at 850 F., ingots about 18 inches thick, 40 to50* inches wide and 12 feet long to the final plate thickness of about 3to 5 inches. The parameters for homogenization prior to rolling togetherwith the solution heat treatment and artificial aging parameters werethe same as set forth earlier in this Example 11. The average shorttransverse mechanical properties for this plate were about 1.2%elongation and 82 in. lb./sq. in. unit propagation energy in tearresistance. The plate did, however, exhibit relatively satisfactoryshort transverse resistance to stress corrosion in the acceleratedalternate immersion tests.

In a further test the fabrication cycle was altered so that the ingotwas compressed in the 18 inch direction at a temperature of 850 to 900F. by drawing down in a forging press to form an elongated slab about 10inches thick. The forged slab was reheated and hot rolled at about 850F. to form the plate, the other steps being the same as with theconventionally hot rolled plate. This forging step did not impart anyconsistent or significant improvement in short transverse properties. Inaddition, this plate was found to exhibit generally inadequateresistance to short transverse stress corrosion in that few, if any,specimens survived the accelerated alternate immersion test for morethan thirty days, many specimens failing in as little as three to tendays and at stress levels considerably below 50% yield. Additionalplates of the same composition were produced in accordance with theforging and rolling procedure as taught by this invention. For thispurpose, round ingots were cast, cropped and scalped to yield stockabout inches in length and 37 inches in diameter. The ingots were upsetat 800 to 900 F. in the A direction to form biscuits about 30 to 32inches high which were reheated to 850 F. and drawn down into elongatedslabs about 14 inches thick. These slabs were hot rolled at 850 F. toform the plate which was stretched, solution heat treated andartificially aged as set forth previously in this Example II. This plateexhibited the same markedly improved short transverse mechanicalproperties as did the plate first cited in this example which also wasfabricated in accordance with the forging practice of the invention.However, when short transverse specimens from this plate were subjectedto the accelerated alternate immersion tests, they exhibited significantsusceptibility to failure by stress corrosion cracking. Many sampleswere found to fail in as short a time as 3 days and even less, and atapplied stress levels about as low as 35% of the yield strength and evenlower. Thus, both the critical fabrication cycle and the criticalcomposition control described herein are necessary to yield satisfactoryplate in accordance with the invention. Failure to follow either or bothwill result in inadequate short transverse properties or inadequateresistance to stress corrosion resistance or both.

At this point it may be mentioned that a considerable quantity of platehaving the composition range as set forth at the outset of Example IIhas been produced and that this plate has consistently exhibited thedesired characteristics of mechanical and stress corrosion propertiesand that hence this particular composition is considered preferredwithin the practice of the invention.

The invention has been described with particular reference to preferredembodiments; however, the invention is not limited to such. Variousminor modifications will suggest themselves to those skilledin the artand it is intended to cover in the appended claims all suchmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. The method of producing improved aluminum base alloy platecomprising:

(1) providing an elongated body composed of an alloy consistingessentially of aluminum, 3 to 6% copper, 0.8 to 3% magnesium and 0.3 to1% manganese, the minimum magnesium content being governed by therelation:

Mg min.=0.32 Cu where Mn is not greater than 0.5%, Mg min.=0.2+0.32Cu-0.4 Mn where Mn is greater than 0.5%, the body having a length tobase ratio of at least 2 to 1,

(2) heating said body to a temperature of at least 800" F. for a periodof time sufiicient to homogenize the internal structure of said body,

(3) initially upsetting by forging, at a temperature of about 600 to 900F., said body in an axial direction to effect an A reduction to form abiscuit having a height of not more than 40% of the original bodylength,

(4) further reducing the biscuit at a temperature of about 600 to 900 F.in the A direction to form an elongated slab having a thickness of notmore than of the biscuit height,

(5) hot rolling at a temperature of about 600 to 900 F. said elongatedslab to a final plate thickness of from 1 to 8 inches,

(6) solution heat treating the plate at a temperature of about 800 to950 F. for a period of time sufficient to elfect substantial solution ofthe soluble alloy constituents and quenching the plate, and

(7) artificially aging the plate at a temperature of about 300 to 400 F.for about six to twenty-four hours,

the resulting plate exhibiting improved short transverse elongation,ranging from 3% to 6%, improved short transverse tear resistancecharacterized by a unit propagation energy of at least 100, andsubstantial immunity to stress corrosion in the short transversedirection at a stress of 50% of the yield strength in the shorttransverse direction.

2. The method according to claim 1 wherein the minimum magnesium contentis governed by the relation:

Mg min.=0.2+0.32 Cu where Mn is not greater than 0.5%,

Mg min.=0.4+0.32 Cu0.4 Mn where Mn is greater than 0.5%,

and the plate produced exhibits substantial immunity to stress corrosionin the short transverse direction at a stress of 75% of the yieldstrength in the short transverse direction.

3. The method of producing improved aluminum base alloy platecomprising:

(1) providing an elongated body composed of an alloy 10 consistingessentially of aluminum, 3.5 to 5% copper, 1 to 2% magnesium and 0.3 to1% manganese, the minimum magnesium content being governed by therelation:

Mg min.=0.32 Cu where Mn is not greater than 0.5%, Mg min.=0.2-|-0.32Cu0.4 Mn where Mn is greater than 0.5%, the body having a length to baseratio of at least 2 to l,

(2) heating said body to a temperature of at least 800 F. for a periodof time sufficient to homogenize the internal structure of said body,

(3) initially upsetting by forging at a temperature of about 600 to 900B, said body in an axial direction to effect an A reduction to form abiscuit having a height of not more than 40% of the original bodylength,

(4) further reducing the biscuit at a temperature of about 600 to 900 F.in the A direction to form an elongated slab having a thickness rangingfrom A to /2 of the biscuit height,

(5) hot rolling said elongated slab at a temperature of about 600 to 900F. to a final plate thickness of from 2 inches to 6 inches,

( 6) solution heat treating the plate at about 800 to 950 F. for aperiod of time suificient to effect substantial solution of the solublealloy constitutents and quenching said plate,

(7) mechanically stress relieving the plate by stretching atsubstantially room temperature to effect a 1%% to 3% permanent stretch,and

(8) artificially aging the plate at about 300 to 400 F.

for about six to twenty-four hours, the resulting plate exhibitingimproved short transverse elongation, ranging from 3% to 6%, improvedshort transverse tear resistance characterized by a unit propagationenergy of at least 100, and substantial immunity to stress corrosion inthe short transverse direction at a stress of 50% of the yield strent ghin the short transverse direction. N

- 4. The method according to claim 3 wherein the minimum magnesiumcontent is governed by the relation:

Mg min.-=0.2+0.32 Cu where Mn is not greater than Mg min.=0.4+0.32 Cu0.4Mn where Mn is greater than 0.5%,

and the plate produced exhibits substantial immunity to stress corrosionin the short transverse direction at a stress of 75% of the yieldstrength in the short transverse direction.

5. The method according to claim 3 wherein the plate produced ranges inthickness from 1 inch to 8 inches.

6. The method according to claim 3 wherein the length to base ratio ofthe alloy body is at least 2% to 1 and the height of the biscuit is notmore than 25% of the original alloy 'body length.

7. The method of producing improved aluminum base alloy platecomprising:

(1) providing an elongated body composed of alloy consisting essentiallyof aluminum, 4.15 to 4.60% copper, 1.45 to 1.65% magnesium, 0.5 to 0.65%manganese, up to 0.05% titanium and up to 0.002% boron, and thefollowing maximum.- impurity levels: silicon 0.15%, iron 0.25%, chromium0.10%, zinc 0.20% and nickel 0.05 the minimum magnesium content beinggoverned by the relation:

Mg Min.=0.2-|-,0.32 Cu0.4 Mn, the body having a length to base ratio ofat least 2 to l,

(2) heating said body to a temperature of about 900 to 925 F. for aperiod of time sufficient to homogenize the internal structure of saidbody,

(3) initially upsetting by forging said body at a temperature of about600 to 900 F. in axial direction 11 to effect an A" reduction to form abiscuit having a height of not more than 40% of the original bodylength,

(4) further reducing the biscuit in the A direction at a temperature ofabout 600 to 900 F. to form an elongated slab having a thickness rangingfrom A to /2 of the biscuit height,

(5) hot rolling said elongated slab at a temperature of about 600 to 900F. to a final plate thickness of from 2 inches to 6 inches,

(6) solution-heat treating the plate at about 800 to 950 F. for a periodof time sufiicient to effect substantial solution of the soluble alloyconstituents and quenching said plate,

(7) stretching said plate at room temperature to effect a permanentstretch of 1 /2% to 3%, and

(8) artificially aging the plate at about 360 to 390 F.,

for about six to twenty-four hours,

the resulting plate exhibiting improved short transverse elongation,ranging from 3% to 6%, improved short transverse tear resistancecharacterized by a unit propagation energy of at least 100, andsubstantial immunity to stress corrosion in the short transversedirection at a stress of 50% of the yield strength in the shorttransverse direction.

8. The method as in claim 7 wherein the minimum magnesium content isgoverned by the relation:

Mg min. =0.4+0.32 Cu0.4 Mn, and the plate produced exhibits substantialimmunity to short transverse stress corrosion at stress levels up to andexceeding 75% of the alloy yield strength.

9. The method as in claim 7 wherein the length to base ratio of thealloy body is at least 2 /2 to 1 and the height of the biscuit is notmore than 25% of the original alloy body length.

10. Improved aluminum base alloy plate ranging in thickness from 2 to 6inches and composed of an alloy consisting essentially of aluminum, 3 to6% copper, 0.8 to 3% magnesium, 0.3 to 1% manganese, the minimummagnesium content being governed by the relation:

12 Mg min. =0.32 Cu where Mn is not greater than 0.5%, Mg min. =0.2+0.32Cu0.4 Mn where Mn is greater than 0.5%,

5 the plate having an internal structure produced by:

(1) heating to a temperature over 800 F, for a period of time sufficientto homogenize the internal structure,

(2) initially upsetting by forging at a temperature of about 600 to 900F. a body of said alloy having a length to base ratio of at least 2 to1, to effect an A reduction to form a biscuit having a height of notmore than 40% of the original body length,

(3) further reducing the resulting biscuit at a temperature of about 600to 900 F. in the A direction to form an elongated slab having athickness of not more than A of the biscuit height,

(4) hot rolling said elongated slab at a temperature of about 600 to 900F. to form said plate,

(5) solution heat treating the plate at a temperature of about 800 to950 F. for a period of time sufficient to effect substantial solution ofthe soluble alloy constituents and quenching said plate, and

(6) artificially aging the plate at a temperature of about 300 to 400 F.for about six to twenty-four hours,

the plate exhibiting improved short transverse elongation, ranging from3 to 6%, improved short transverse tear resistance characterized by aunit propagation energy of at least 100, and substantial immunity tostress corrosion in the short transverse direction at a stress of 50% ofthe alloy yield strength in the short transverse direction.

3/1954 Rosenkranz 148--12.7 X 8/1966 Foerster 14832.5 X

DAVID L. RECK, Primary Examiner.

CHARLES N. LOVELL, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,333,989 August 1 1967 Melvin H. Brown t 1 It is hereby certified that errorappears in the above-numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 1 line 13 for "0 .3" read .3 column 2, line 13, for "the alloytype described, this technique has produced" read the thickness of aconventional ingot from about 16 inches line 15 for "thea lloy" read thealloy colum 7 line 35 for "substantail" read substantial column 10 line28 for "constitutents" read constituents Signed and sealed this 25th dayof June 1968 (SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

1. THE METHOD OF PRODUCING IMPROVED ALUMINUM BASE ALLOY PLATECOMPRISING: (1) PROVIDING AN ELONGATED BODY COMPOSED OF AN ALLOYCONSISTING ESSENTIALLY OF ALUMINUM, 3 TO 6% COPPER, 0.8 TO 3% MAGNESIUMAND 0.3 TO 1% MANGANESE, THE MINIMUM MAGNESIUM CONTENT BEING GOVERNED BYTHE RELATION: MG MIN.=0.32 CU WHERE MN IS NOT GREATER THAN 0.5%, MGMIN.=0.2+0.32 CU-0.4 MN WHERE MN IS GREATER THAN 0.5%, THE BODY HAVING ALENGTH TO BASE RATIO OF AT LEAST 2 TO
 1. (2) HEATING SAID BODY TO ATEMPERATURE OF AT LEAST 800*F. FOR A PERIOD OF TIME SUFFICIENT TOHOMOGENIZE THE INTERNAL STRUCTURE OF SAID BODY, (3) INITIALLY UPSETTINGBY FORGING, AT A TEMPERATURE OF ABOUT 600* TO 900*F., SAID BODY IN ANAXIAL DIRECTION TO EFFECT AN "A" REDUCTION TO FORM A BISCUIT HAVING AHEIGHT OF NOT MORE THAN 400*F., OF THE ORIGINAL BODY LENGTH, (4) FURTHERREDUCING THE BISCUIT AT A TEMPERATURE OF ABOUT 600* TO 900*F. IN THE "A"DIRECTION TO FORM AN ELONGATED SLAB HAVING A THICKNESS OF NOT MORE THAN3/4 OF THE BISCUIT HEIGHT, (5) HOT ROLLING AT A TEMPERATURE OF ABOUT600* TO 900* F. SAID ELONGATED SLAB TO A FINAL PLATE THICKNESS OF FROM 1TO 8 INCHES, (6) SOLUTION HEAT TREATING THE PLATE AT A TEMPERATURE OFABOUT 800* TO 950*F. FOR A PERIOD OF TIME SUFFICIENT TO EFFECTSUBSTANTIAL SOLUTION OF THE SOLUBLE ALLOY CONSTITUENTS AND QUENCHING THEPLATE, AND (7) ARTIFICIALLY AGING THE PLATE AT A TEMPERATURE OF ABOUT300* TO 400*F. FOR ABOUT SIX TO TWENTY-FOUR HOURS, THE RESULTING PLATEEXHIBITING IMPROVED SHORT TRANSVERSE ELONGATION, RANGING FROM 3% TO 6%,IMPROVED SHORT TRANSVERSE TEAR RESISTANCE CHARACTERIZED BY A UNITPROPAGATION ENERGY OF AT LEAST 100, AND SUBSTANTIAL IMMUNITY TO STRESSCORROSION IN THE SHORT TRANSVERSE DIRECTION AT A STRESS OF 50% OF THEYIELD STRENGTH IN THE SHORT TRANSVERSE DIRECTION.